Apparently whenever Firefox releases an update, the folder name that contains your profile changes. This may change the pathnames for some user-defined backups you set up prior to upgrading. For instance, I had created a backup entry for FireFTP as:

but after upgrading to the latest version of Firefox, the backup did not work. While researching the problem I discovered the 8 characters that precede the profile name (the "salt" value") changed after the upgrade and the FireFTP data now resided at